The use of high intensity light sources such as lasers for cutting and reshaping eyes has expanded in recent years in part due to the superior precision, controllability and safety which such cutting technology offers over other cutting technologies, such as mechanical cutting of the eye. One type of ophthalmic surgical procedure for which high-intensity light radiation is particularly well suited is the radial keratotomy procedure in which a number of radial incisions are made on the cornea of the eye to change the curvature of the cornea.
Several methods and apparatus for performing radial keratotomies with lasers have been proposed. See, for example, U.S. Pat. No. 4,648,400 to Schneider et al; and U.S. Pat. No. 4,665,913 to L'Esperance, Jr. Schneider et al describe the use of lasers to selectively ablate the cornea of the eye by directing the laser beam through a generally planar mask having radial slots. The radial slots of the mask permit portions of the laser beam to pass through the mask and incise the cornea in a pattern of circumferentially spaced radial incisions.
Lasers have also been used to ablate an annular portion of the cornea by scanning or variably attenuating the laser beam. Such scanning changes the front surface of the cornea to a different optical curvature, thereby changing the refraction of the eye. See, for example, U.S. Pat. No. 4,669,466 to L'Esperance.
In such an application, it is desirable to deliver uniform beam energy along the curved scanning path. However, since the cornea presents a convexly curved surface to the laser beam, the outer circumferential portions of the cornea lie at further distances from the beam source than those portions at or near the center of the cornea. Thus, the laser beam incidents the cornea with a different angle along the cornea's constantly changing surface which causes variation of the energy density of the laser beam in a direction perpendicular to the corneal surface.
Using a mask to produce corneal incisions does not focus the beam on the corneal surface but merely projects the beam toward the surface.
Additionally, the energy of the laser beam may not be distributed uniformly due to the position of the mask relative to the beam or due to inherent non-uniformity of the laser beam itself. A non-uniform distribution of energy results in differing depths of the radial incisions, leading to an improper restructuring of the curvature of the cornea.
More recently, laser systems have been developed to ablate intrastromal areas of a cornea without ablating or piecing the external surface of the cornea. These laser systems focus the lower beams as a spot on the intrastromal areas of the cornea to be ablated. Thus, to create a linear incision, the laser must ablate a plurality of spots along the line of incisions. An example of a laser system employing spot focus ablation is disclosed in U.S. Pat. No. 4,907,586 to Bille et al, which is hereby incorporated herein by reference.